Euclidean quotients of finite metric spaces

This paper is devoted to the study of quotients of finite metric spaces. The basic type of question we ask is: Given a finite metric space M and α?1, what is the largest quotient of (a subset of) M which well embeds into Hilbert space. We obtain asymptotically tight bounds for these questions, and prove that they exhibit phase transitions. We also study the analogous problem for embeddings into ?_p, and the particular case of the hypercube.     

Euclidean embeddings of finite metric spaces

This is a brief survey on Euclidean embeddings of finite metric spaces, focusing on the power transform metric with many examples. Some old results are presented in slightly improved forms, and the last section contains a few new results. Proofs are given if they are elementary and not too long. Several problems and conjectures are also given.     

On the boundary behavior of imbeddings of metric spaces into a Euclidean space

We investigate the boundary behavior of so-called Q-homeomorphisms with respect to a measure in some metric spaces. We formulate a series of conditions for the function Q(x) and the boundary of the domain under which any Q-homeomorphism with respect to a measure admits a continuous extension to a boundary point. __________ Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, No. 8, pp. 1068–1074, August, 2007.     

Duality of metric entropy in Euclidean space

Let K be a convex body in a Euclidean space, K° its polar body and D the Euclidean unit ball. We prove that the covering numbers N(K,tD) and N(D,tK°) are comparable in the appropriate sense, uniformly over symmetric convex bodies K, over t>0 and over the dimension of the space. In particular this verifies the duality conjecture for entropy numbers of linear operators, posed by Pietsch in 1972, in the central case when either the domain or the range of the operator is a Hilbert space. To cite this article: S. Artstein et al., C. R. Acad. Sci. Paris, Ser. I 337 (2003).     

Isoperimetric inequalities of Euclidean type in metric spaces

No Abstract. .Received: August 2003 Revision: June 2004 Accepted: August 2004     

On the optimal stability of bases of univariate functions

Summary. This paper is concerned with bases of finite dimensional spaces of univariate continuous functions which are optimally stable for evaluation. The only bases considered are those whose elements have no sign changes. Among these, an optimally stable basis is characterized under the assumption that the set of points where each basis function is nonzero is an interval. A uniqueness result and many examples of such optimally stable bases are also provided. Received May 26, 2000 / Published online August 17, 2001     

Diffusive stability for periodic metric graphs

We consider a nonlinear diffusion equation on an infinite periodic metric graph. We prove that the terms which are irrelevant w.r.t. linear diffusion on the real line are irrelevant w.r.t. linear diffusion on the periodic metric graph, too. The proof is based on L¹‐ estimates combined with Bloch wave analysis for periodic metric graphs.     

On the stability of wavelet and Gabor frames (Riesz bases)

If the sequence of functions _{j, k} is a wavelet frame (Riesz basis) or Gabor frame (Riesz basis), we obtain its perturbation system _j,k which is still a frame (Riesz basis) under very mild conditions. For example, we do not need to know that the support of or is compact as in [14]. We also discuss the stability of irregular sampling problems. In order to arrive at some of our results, we set up a general multivariate version of Littlewood-Paley type inequality which was originally considered by Lemarié and Meyer [17], then by Chui and Shi [9], and Long [16].     

Homogeneous and rank bases in spaces of metric configurations

Two families of sets of metric configurations are considered. The conditions are established under which sets from these families are bases for a special linear vector space. It is shown that the transition from the representation of a metric configuration in the trivial basis to its representation in any of the considered bases and back can be effectively calculated. It is shown that the nonnegativity of the decomposition of a metric configuration in the considered bases is a sufficient condition for the semi-metric axioms to hold for this configuration, while the nonnegativity of the decomposition in a rank basis is a necessary and sufficient condition for the metric configuration to have a given rank. The transition coefficients and decomposition components are interpreted in the case of homogeneous bases. Sets from the considered families are indicated that characterize largest-volume cones of metric configurations.     

Model theoretic stability and categoricity for complete metric spaces

We deal with the systematic development of stability in the context of approximate elementary submodels of a monster metric space, which is not far, but still very different from first order model theory. In particular, we prove the analogue of Morley’s theorem for classes of complete metric spaces.     

On the heat flow on metric measure spaces: existence, uniqueness and stability

We prove existence and uniqueness of the gradient flow of the Entropy functional under the only assumption that the functional is λ-geodesically convex for some ${\lambda\in\mathbb {R}}$ . Also, we prove a general stability result for gradient flows of geodesically convex functionals which Γ?converge to some limit functional. The stability result applies directly to the case of the Entropy functionals on compact spaces.     

Symmetry properties of resolving sets and metric bases in hypercubes

In this paper we consider some special characteristics of distances between vertices in the \(n\)-dimensional hypercube graph \(Q_n\) and, as a consequence, the corresponding symmetry properties of its resolving sets. It is illustrated how these properties can be implemented within a simple greedy heuristic in order to find efficiently an upper bound of the so called metric dimension \(\beta (Q_n)\) of \(Q_n\), i.e. the minimal cardinality of a resolving set in \(Q_n\). This heuristic was applied to generate upper bounds of \(\beta (Q_n)\) for \(n\) up to \(22\), which are for \(n\ge 19\) better than the existing ones. Starting from these new bounds, some existing upper bounds for \(23\le n\le 90\) are improved by a dynamic programming procedure.     

On the reduction to a subspace of stability properties of systems in metric spaces

Summary The objects studied are dynamical and semidynamical systems defined on arbitrary metric spaces. Sufficient conditions for stability and asymptotic stability (both local and global) of a compact invariant set are established, using the corresponding properties of the (semi-) dynamical systems induced on a (positively) invariant subset.